A conventional fixing device is provided with an endless belt including a base layer made from resin or metal and an elastic layer made from rubber formed over an outer surface of the base layer. The fixing device is also provided with a restriction member positioned outward of widthwise end surfaces of the endless belt. The widthwise end surfaces are positioned at ends of the belt in a widthwise direction perpendicular to a circumferential direction of the belt. The widthwise end surfaces may be slidingly contacted with the restriction member in a circular movement of the endless belt. Therefore, widthwise end surfaces of the elastic layer are also in sliding contact with the restriction member, which causes resistance against circular movement of the endless belt, and may cause frictional wearing of the elastic layer.
Japanese Patent Application Publication No. 2008-185661 discloses a fixing device provided with a lubricant retaining portion. The retaining portion is provided at each widthwise end surface of the endless belt. Lubricant retained in the lubricant retaining portion can lower sliding resistance between the widthwise end surfaces of the endless belt and the restriction member.